Return 202 Accepted for SSE responses per MCP spec

`StreamableHTTPTransport#send_response_to_stream` returned 200 with
`{ accepted: true }` body, but the MCP spec requires 202 with empty body
when a response is sent via SSE stream. This has been a requirement since
Streamable HTTP was introduced in protocol version 2025-03-26.

- https://modelcontextprotocol.io/specification/2025-03-26/basic/transports#sending-messages-to-the-server
- https://modelcontextprotocol.io/specification/2025-11-25/basic/transports#sending-messages-to-the-server

lib/mcp/server/transports/streamable_http_transport.rb

@@ -288,7 +288,7 @@ module MCP
         def send_response_to_stream(stream, response, session_id)
           message = JSON.parse(response)
           send_to_stream(stream, message)
-          [200, { "Content-Type" => "application/json" }, [{ accepted: true }.to_json]]
+          handle_accepted
         rescue IOError, Errno::EPIPE => e
           MCP.configuration.exception_reporter.call(
             e,